Electrical switching devices are well known in the field of medium and high voltage switching applications. They are e.g. used for interrupting a current when an electrical fault occurs. As an example, circuit breakers filled with a dielectric insulation fluid have the task of opening contacts and keeping them far apart from one another in order to avoid a current flow, even in case of high electrical potential originating from the electrical fault itself.
Dielectric insulating fluids are widely applied to insulate an electrically active part in a variety of electrical switching devices such as gas-insulated switchgear (GIS). For example, the electrically active part in medium or high voltage metal-encapsulated switchgear is arranged in a gas-tight compartment which encloses an insulation gas with a pressure of several bars, which electrically separates the compartment of the apparatus from its electrically active part. In other words, the insulation gas does not allow the flow of electrical current from the electrically active part to the compartment. A commonly used dielectric insulation gas is sulfur hexafluoride (SFg) which exhibits excellent dielectric insulation. and electric are extinguishing capabilities. However, SF6 is a strong contributor to the green-house effect and thus has a high global warming potential. Therefore, manufacturers of GIS have striven for alternative insulation fluids. Several alternative insulation fluids have been identified. Some of these alternatives comprise multi-component fluid mixtures, i.e. they comprise more than one molecular or atomic species. It is found that certain properties of such insulation fluid mixtures are compulsory to the safe operation of the electrical apparatus. As an example, the dielectric break-down strength of the insulation fluid is strongly dependent on local concentration ratios of the mixture fluid components and on total fluid pressure.
Among the proposed candidates for SF6-replacement, the fluoroketones (FKs) with 5 or 6 carbon atoms (c5, c6) mixed with carrier gases appear to be most promising. FKs are reported to have good dielectric properties and contrary to SFg, FKs only have a low inspect on global warming and are rapidly degraded in the atmosphere.
Since these insulation media comprise or consist of a mixture of gases, it is necessary to be able to determine the concentration of the various components, in particular that of the FK, in order to ensure safety and functionality of the electrical apparatus.
In addition to the concentration of gas components, there are a number of other relevant aspects and/or properties involved in the process of maintaining and monitoring an electrical switching device, e.g. monitoring humidity accumulation within the compartment, which may as well seriously affect functionality of such an electrical switching device.
In the following the invention will be described for gas insulated switchgear (GIS), which term will therefore be used in the general sense of electrical switching device.